The input data stream of an optical transmission system may be viewed as a series of light pulses representing digital bits. The bit rate of current optical transmission systems generally range from 10 Gbit/s to 40 Gbit/s resulting in light pulses (or bit periods) that are, respectively, 100 to 25 picoseconds (psecs) wide.
Receivers in an optical transmission system convert each bit period in the data stream into digital ones or zeros by determining, for each bit period, whether a light pulse has been received (digital one) or not (digital zero).
Polarization mode dispersion (PMD) is a phenomenon that may distort the light pulses of the data stream and thus impair the ability of a receiver to determine whether a bit period should be converted into a one or zero. As a result, PMD limits the transmission accuracy and capacity of optical transmission systems and therefore is a critical parameter in optical communications. The fundamental mode of a single mode optical fiber is the solution to the wave equation that satisfies the boundary conditions at the core-cladding interface. Although this appears to be counter-intuitive, there are two solutions to the wave equation that correspond to the fundamental mode. The fiber is deemed to be a single mode fiber because both solutions have the same propagation constant. The two solutions are referred to as the polarization modes. These polarization components are mutually orthogonal. The state of polarization refers to the distribution of light energy between the two polarization modes. In practice, since the cross-sectional area of a fiber is not perfectly circular, the two polarization modes have slightly different propagation constants that give rise to pulse spreading. One polarization mode is referred to as the “fast-mode”, and the other polarization mode is known as the “slow-mode”. The fast mode and the slow mode mix as they travel down the fiber, becoming indistinguishable. The resulting difference in propagation time between polarization modes is known as the differential group delay.
Optical signal transmitted through an optical fiber are subjected also to another physical dispersion known as chromatic dispersion (CD). This occurs because each wavelength of the optical signals travels through a given medium, such as an optical fiber, at a different speed. Since the various wavelengths of light have different velocities, a given wavelength of light will arrive at a receiver disposed at the end of a transmission fiber before another wavelength of light will arrive at that receiver. The time delay between different wavelengths of light leads to pulse broadening. Chromatic dispersion is obtained in an optical fiber by measuring fiber group delays in the time domain. Chromatic dispersion is a relatively stable phenomenon. It can be in the range of 300-500 psec in a 10 Gb/s system before incurring a 1 dB power penalty. In a 40 Gb/s system, the range decreases to 18-25 psec.
CD can be time variant as a result of changes with temperature or stress, but typically, the time variance of CD is not particularly strong. PMD, on the other hand, is very time variant, and thus, compensation should track with time. PMD describes the statistical broadening of optical pulses within an optical fiber caused by polarization effects. This broadening effect, similar to pulse broadening from chromatic dispersion, ultimately prevents the correct detection of the waveform at the receiver.
In WO 03/040777 is described an integrated system for performing dispersion compensation on wavelength channels in WDM or DWDM transmissions (D for dense). The system includes a tunable integrated dispersion compensation module that performs chromatic dispersion compensation and polarization mode dispersion compensation on each of the wavelength channels in the transmission. Feedback is used to adjust the tunable integrated dispersion compensation module until receiver performance is optimized. Such an integrated system as described in WO 03/040777 has the big disadvantage to require a specific dispersion compensation module for each different wavelength channels of the optical signals. A dispersion compensation using such solution will imply high costs and therefore can not be really efficient.
In WO 02/080411 is described a method and apparatus for compensation for polarization mode dispersion (PMD) in an optical transmission system without perturbing the laser source. Such optical PMD compensator is rather attractive for bit rates from 10 GHz to 40 GHz WDM optical transmission system. Indeed, the approach is cost-effective due to parallel processing of many channels within one hardware (Liquid crystal display array for polarization controllers) and one birefringent crystal (polarization maintaining fiber) for all parallel wavelength channels. Also only one wavelength scanning polarimeter (feedback signal) is shared among all channels. Furthermore, it is taken advantage of the use of fastfeed-forward adaptation which avoids time consuming dither techniques i.e. consecutive variations of polarization controller tuning parameters into the direction of an optimized feedback signal.
Nevertheless, a solution as proposed in WO 02/080411 has still the big drawback that neither chromatic dispersion (CD) or transmitter shirp nor self-phase modulation (SPM) can be compensated.